Waveguide slot radiator with electronic phase and amplitude control



April 21, 1970 B. J. FORMAN- 3, 0

v WAVEGUIDE SLOT RADIATOR WITH ELECTRONIC PHASE AND AMPLITUDE CONTROL Filed June 7, 1967 2 sheets sheet 1 ATTORNEY.

B. J. FO RMAN 3,508,268 WAVEGUIDE SLOT RADIATOR WITH ELECTRONIC PHASE A ril 21, 1970 AND AMPLITUDE CONTROL 2 Sheets-Sheet 2 Filed June 7, 1967 Fig (zero plasma density 2 J m Y o m m I m IV {R a 1 :1 \O a /V//// Fig. 4.

m m m m w m \t w m ,w i m m m n $3: i I

Barry J. Formon,

INVENTOR.

Fig. 6.

ATTORNEY.

United States Patent O U.S. Cl. 343701 3 Claims ABSTRACT OF THE DISCLOSURE A waveguide slot radiating and coupling device having independently controllable amplitude and phase characteristics. Control of the characteristics of the slot radiating and coupling device is obtained through the use of variable lumped reactance plasma elements employed as irises in the vicinity of the waveguide slot. The variable lumped reactance plasma elements provide low-loss RF control properties and an ability to withstand RF power levels greater than that obtainable with semiconductor devices. In a preferred embodiment a plurality of variable lumped reactance plasma elements are disposed in the vicinity of a slot in a waveguide. Continuous control of the amplitude and phase of the slot coupling or radiation over a wide range is obtained.

FIELD OF THE INVENTION This invention relates to microwave coupling and radiating devices and more specifically to microwave coupling and radiating devices utilizing an electronically-controlled waveguide slot.

DESCRIPTION OF THE PRIOR ART It is well-known that electromagnetic wave energy is not radiated from a longitudinal slot extending along the center line of a rectangular waveguide supporting propagating wave energy in the dominant mode. However, an iris asymmetrically disposed in the waveguide in a plane transverse to the slot creates a modal asymmetry which distorts the electromagnetic field in the vicinity of the slot, causing it to radiate. Thus, by varying the degree of this asymmetrical perturbation, the characteristics of the slot radiation can be controlled.

In the past, metallic plates have been used to form such variable irises for controlling slot radiation from a rectangular waveguide. In such devices one or more irises are disposed in a transverse plane extending outwardly through the narrow Walls of the waveguide, thereby permitting mechanical adjustment. By displacing the iris or irises asymmetrically, a resultant asymmetrical field distribution is established in the wavegulide in the vicinity of the 'slot, and as a result radiation from the slot is obtained. This device is described in detail in U.S. Patent No. 2,946,057, issued to H. E. Shanks on July 19, 1960.

With the above-mentioned approach, it is necessary to physically adjust the iris assembly to vary the slot coupling characteristics. Although well-suited for many applications, it is understandable that the speed with which adjustments of the coupling characteristics can be made is limited.

Accordingly, it is an object of the present invention to provide an improved variable microwave coupling and radiating device capable of high-speed operation.

.Previous attempts to secure high-speed operation with devices of this general type have been made. In one such device, the two plates of an iris are fabricated of ferrite material and subjected to a magnetic field provided by externally mounted electromagnets. With this device the ferrite iris is permanently positioned within the waveguide and the slot coupling is controlled by an appropriate adjustment of the relative magnetic biasing field applied to the ferrite material. A more complete description of this device is found in U.S. Patent No. 2,946,056, also granted to H. E. Shanks on July 19, 1960.

Again, although the ferrite-iris structure mentioned above is suitable for many applications, and provides superior operation in some instances, it may be unsuitable for other applications. For example, in applications where weight, size, ambient temperature sensitivity, hysteresis and available driving power are important limitations, the ferrite-iris structure may be unsuitable. In particular, the size of the electromagnets can prevent the close inter-element spacing Where a plurality of slots are used as elements of a phased array.

It is therefore another object of the present invention to provide a light-Weight, compact, non-hysteresis, variable microwave coupling and radiating device requiring relatively low driving power.

Yet another electronically variable slot radiator is described in U.S. Patent No. 3,266,043, granted to F. J. Goebels, Jr., on Aug. 9, 1966. In this device slot radiation control is obtained through the use of semiconductor elements mounted within the waveguide in the vicinity of the centrally disposed slot. Controlled radiation is obtained by adjusting the relative biasing potential on the semiconductor elements.

The semiconductor iris structure, while overcoming many of the above-mentioned limitations of the mechanical iris and ferrite iris configurations, lacks sufficient power handling capability for high-power applications. Even accidental excursions above the power handling capabilities of the semiconductor elements may result in permanent damage to the semiconductor elements or in changes in their characteristics.

Accordingly, it is yet another object of the present invention to provide an improved variable microwave coupling and radiating device capable of operating at relatively high-power levels.

SUMMARY OF THE INVENTION In accordance with the principles of the present invention, the above objects are accomplished by the use of variable irises of the variable lumped reactance gas plasma variety. In a preferred embodiment a section of rectangular, conductively-bounded waveguide is provided with an elongated slot extending through one of its broad walls in a direction parallel to the longitudinal axis of the guide. Preferably, the slot is located in the center of the wall so that it is ordinarily non-radiating; that is, wave energy propagating through the guide in the dominant TE mode is not coupled to the slot. At least one pair of variable lumped reactance plasma elements is disposed in the waveguide in the vicinity of the slot, one gas plasma element of each pair being disposed on either side of the slot.

By the use of appropriate adjustable voltage biasing means the inductive reactance presented to the wave energy propagating through the guide by each gas plasma element can be adjusted. By such adjustment asymmetrical wall currents are set up in the vicinity of the slot causing the wave energy to be coupled thereto. If the device 5 is to be used as a variable coupling arrangement, a second section of conductively-bounded waveguide can be disposed contiguous to the first waveguide section adjacent the slot and electromagnetically coupled thereto for propagating the wave energy coupled through the slot.

BRIEF DESCRIPTION OF THE DRAWINGS These and other objects and advantages of the present invention will be more readily understood by reference to the following description taken in conjunction with the accompanying drawings, in which like numerals refer to like elements and in which:

FIG. 1 is a pictorial view, partially broken away, of an embodiment of the present invention;

FIG. 2 is a cross-sectional view of the embodiment of FIG. 1;

FIGS. 3 and 4 are plan views of a portion of the embodiment of FIG. 1, illustrating the wall current distribution in the vicinity of the slot;

FIG. 5 is a plan view of a portion of another embodiment of the present invention; and

FIG. 6 is a pictorial view of yet another embodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring more specifically to the drawings, FIG. 1 is a pictorial view of a first embodiment of the present invention. In the embodiment of FIG. 1 a section of conductively bounded, rectangular waveguide 10 is provided with first and second coupling flanges 11 and 12 disposed at either end thereof. An elongated resonant slot 13 extends through the top broad wall of waveguide section 10 in a direction substantially parallel to the longitudinal axis of the waveguide. Preferably, slot 13 is located midway between the narrow walls of waveguide section 10 and proportioned so that it is resonant at the mid-frequency of the frequency band of intended operation.

An ionizable gas, such as neon, xenon or krypton, to mention but a few, is confined at a relatively low pressure within waveguide section 10. Transparent microwave windows 14, only one of which is shown, serve to seal off the slot and open ends of the waveguide for this purpose.

A pair of gas-tight structures indicated by cylinders =15 and 16 are mounted external to waveguide section 10 on the lower broad wall thereof. Structures 15 and 16 are composite structure which house the electrodes for producing the variable reactance plasma columns and serve as RF chokes for the microwave energy propagating within waveguide section 10.

An understanding of the operation of the present invention can be obtained with the aid of the cross-sectional view of FIG. 2. In FIG. 2 a pair of voltage-operated gas plasma devices is shown in simplified form. Each of the gas plasma elements 15 and 16 comprises a cathode electrode 20, a grid control electrode 21 and an anode electrode 22. In general, the anode electrodes can comprise the inner conductive surface of waveguide section 10. In general, conductive means such as lead wires 23, 24 and 25 can be used for connecting the cathode, grid control and anode electrodes, respectively, to the sources of electric potential, not shown. Suitable insulating means such as annular dielectric spacers 26 are employed to conductively insulate grid control electrodes 21 from waveguide section 10. By the same token, cathode electrodes can also be provided with similar insulated mounting means not shown.

Enclosing the above-mentioned electrodes are cylindrical conductive structures 15 and 16. As mentioned hereinabove, structures 15 and 16 serve to confine the ionizable gas within waveguide section 10 and to provide a quarterwave RF choke, not shown, for the microwave energy propagating therein. Although conductive cylindrical structures 15 and 16 are shown partially broken away in FIG. 2, it should be apparent that each is provided with terminating end plate through which lead wires 23, 24, and may pass.

A thin dielectric window 14 fabricated, for example, of quartz is shown disposed adjacent slot '13 in the crosssectional view of FIG. 2. As noted above, the dielectric windows 14 allow the substantially reflectionless transmission of microwave energy while sealing the waveguide section from gas leakage.

In FIG. 2, each of the voltage-operated gas plasma elements is indicated as utilizing a cold cathode. It is obvious, however, that in some applications a hot cathode structure may be desirable. In such instances directly or indirectly headed cathode electrodes can be substituted for cold cathode electrodes 20. A detailed description of several embodiments of a voltage-operated gas plasma element suitable for use in the present invention is given in the co-pending application of B. J. Forman, R. C. Knechtli and J. Y. Wada, Ser. No. 589,120 filed Oct. 24, 1966 now U. S. Patent No. 3,439,297 issued Apr. 15, 1969. Since the analysis of the operation of these devices is given in detail in the above-mentioned copending application, it will be outlined only briefly in the context of the present invention.

In the operation of the variable lumped reactance plasma element, electrons produced at cathode electrode 20 are injected into the gas filling waveguide section 10. As described in the above-cited co-pending application, the energies of the injected electrons are somewhat greater than the ionization potential of the gas. As the electronneutral collisions occur within the gas a localized plasma column results. By controlling the voltages of the grid control electrode 21 and cathode electrode 20 relative to the anode electrode 22 the plasma density, and therefore the reactance of the plasma column, can be controlled. The plasma column can be regarded, therefore, as a variable lumped reactance shunting the waveguide section.

It has been found that an axially applied D.C. magnetic field helps to confine the plasma in a well-defind column and decreases the discharge power required for a given plasma density. In general, this D.C. magnetic field on the order of to 500 oersteds can be provided by one or two small permanent magnets mounted externally alongside the waveguide section 10. If desired, it is also possible to integrate these magnets into the device by the use of magnetic materials for the walls of Waveguide section 10.

The mechanism by which the microwave energy within waveguide section 10 is coupled to slot 13 can be illustrated with the aid of the partial plan views of FIGS. 3 and 4. In FIG. 3 there is shown a plan view of the embodiment of FIG. 1 illustrating the current streamlines in the broad Wall of waveguide section 10 for zero plasrna'dens ity. As mentioned hereinabove, wave energy is proga'gated through waveguide section 10 preferably in the dominant TE mode. The current streamlines set up in the broad wall of waveguide section 10 as a result of this energy are indicated by dashed arrows 30 in FIG. 3. The symmetrical wall current distribution shown in FIG. 3 is typical of the non-radiative mode. That is, due to the symmetry of wall current 30 about the center line of waveguide section 10 the net potential across slot 13 is zero and there is no coupling between the slot and the wave energy within the guide. 1 y

In the plan view of FIG. 4 a plasma column 41 provided by one of the variable reactance plasma elements is established. Due to the reactive iris effect of this element, a modal asymmetry is established in the wave energy propagating through guide 10. The wall currents resulting from this modal asymmetry are shown by dashed arrows 40 By virtue of this asymmetrical current distribution in the Wall of waveguide section 10, a radiative electric field is set up across the narrow dimension of slot 13.

Depending upon the plasma density of the variablereactance plasma elements a greater or lesser degree of coupling is achieved. That is, if the plasma density is very large a greater modal asymmetry results and an electric field of higher intensity exists across the slot than for small plasma densities. Thus, the degree of coupling or the degree of resulting radiation from the slot is varied by the plasma density of plasma column 41.

By biasing the variable reactance plasma elements differentially the shunt reactance of one element with respect to the other is varied, distorting the wall current distribution in the vicinity of the slot accordingly. The slot is thereby coupled to the wave energy Within the guide in a continuously variable fashion.

It should be pointed out that either one of the variable reactance plasma elements can be disposed in waveguide section 10 in other than a symmetrical location with respect to the guide center line; similarly, two elements may be disposed in the guide on the same side of slot 13. That is, although preferred, it is not essential that the gas plasma elements be symmetrically disposed in the waveguide about its longitudinal center line or that the slot be the shunt slot 13 centrally positioned in the waveguide wall since, by proper biasing of the variable reactance plasma elements an asymmetrical configuration may, ineffect, be made to appear symmetrical and vice versa to the electromagnetic wave energy propagating through the waveguide.

In the plan view of FIG. 5, which is similar to the plan views of FIGS. 3 and 4, a centrally disposed slot 50 is provided with four variable lumped reactance plasma elements indicated by the dashed circular outlines 51, 52, 53 and 54. The operation of the embodiment of FIG. 5 is similar to that of FIG. 1, except that it is possible to obtain a continuously variable phase of the wave energy coupled to the slot, in addition to continuously variable amplitude. If even greater latitude in phase and amplitude control is desired it is readily apparent that a greater number of voltage-operated gas plasma elements can be employed.

In FIG. 6 there is shown a pictorial view of another embodiment of the present invention Where in the microwave energy radiated from the slot is coupled to a section of conductively bounded waveguide. In FIG. 6 a first section of conductively bounded rectangular waveguide 60 is provided with end flanges 61 and 62. A second section of rectangular conductively bounded Waveguide 63 abuts against and is mechanically joined to waveguide section 60 'at a right angle. Abutting waveguide section 63 is electromagnetically coupled to a longitudinal slot 64 in the upper broad wall of waveguide section 60. Waveguide section 63 is aso provided with a flange 65 which facilitates coupling to an appropriate utilization device not shown. It is to be noted that the sealed slot 13, 14 in FIG. 2 could be used alternatively in place of waveguide section 60 in order to convert the waveguide slot coupler device to a waveguide slot radiator device.

As in the embodiment of FIG. 1, the open ends of waveguide sections 60 and 63 are provided with suitable dielectric windows 66 for confining the ionizable gas within the structure. Also, in the embodiment of FIG. 1, one or more variable reactance plasma elements are disposed in the vicinity of the slot and adapted to create a modal asymmetry in the wave energy propagating in waveguide section 60. A unitary structure containing the RF chokes and the heating and electron-emitting electrodes of the gas plasma elements is indicated by cylinder 67. The conductive means for applying the operating voltages and currents to the plasma element electrodes can be provided by means of terminals 68 extending through an end plate in conductive cylinder 67 In operation, electromagnetic wave energy is applied to one of the open ends of waveguide section 60, preferably in the dominant TE mode. By differentially biasing the variable reactance plasma devices, the degree of coupling and phase shift between this wave energy, the slot, and hence, Waveguide section 63 can be continuously varied. The coupled wave energy can then be extracted from the open end of waveguide section 63. If desired, a non-reflective load impedance or other utilization means can be provided at the output end of waveguide section 60'.

In one experimental model constructed in accordance with the embodiment of FIG. 6 and utilizing four variable lumped reactance plasma devices as in FIG. 5, the following data were obtained.

TABLE Frequency: X-band Maximum coupling: 8.8 db (0 phase shift); -13 db (0 to 360 phase shift) Phase shift: 0 to 360 continuous to 13 db coupling VSWR variation: 1..35:1 to 1.73:1

RF losses: 2.0 to 8.5 percent Neon at approximately 0.4 torr pressure was utilized as the ionizable gas.

In all cases it is understood that the above-described embodiments are merely illustrative of but a small number of the many possible specific embodiments which can represent applications of the principles of the present in vention. Numerous and varied other arrangements including linear and planar arrays of a plurality of controlled slots can be readily devised in accordance with these principles by those skilled in the art without departing from the spirit and scope of the invention.

What is claimed is:

1. An electronically controlled radiating device comprising, in combination:

a section of rectangular conductively bounded waveguide, one end of said waveguide section being adapted for connection to a source of electromagnetic wave energy;

a slot extending through a broad wall of said wave guide section;

an ionizable gas;

means for confining said ionizable gas within said waveguide section;

at least two independently variable reactance means extending between the broad walls of said waveguide section adjacent said slot;

each of said variable reactance means including a source of free electrons, means for injecting said electrons into said waveguide section at an energy level greater than the ionization potential of said gas and means for controlling the energy level of said injected electrons.

2. The radiating device according to claim 1, having two variable reactance means symmetrically disposed on each side of said slot.

3. The radiating device according to claim 1, having a single variable reactance means symmetrically disposed on each side of said slot.

References Cited UNITED STATES PATENTS 2,577,118 12/1951 Fiske 333-99 3,015,822 1/1962 Brown et al. 343771 3,262,118 7/1966 Jones 343701 3,307,194 2/1967 Shcleg ct al 343l ELI LIEBERMAN, Primary Examiner U.S. Cl. X.R. 

